Ultraviolet (UV) cured acrylic adhesives have been used in various applications including the joining of transparent materials and coatings. Blue light (with a wavelength of approximately 400-475 nm) may used in lieu of ultraviolet to cure materials, and has particularly been used in dental applications and other applications where ultraviolet light might damage tissues. Light emitting diodes provide convenient, small, and intense light sources and are increasingly available with wavelengths in the blue range.
It is possible to induce an acrylate cross-linking reaction by light at wavelengths up into the near IR. There were some early holographic technologies for data storage based on laser diodes running at around 750-800 nm wavelength in combination with IR-sensitive dyes. However, quantum efficiencies are not high at these long wavelengths and greatly improve as the photon energy increases. UV curing is often done with lamps of fairly low brightness but with high quantum efficiency.
Blue light curing of dental composite materials has been used for 15 years. Mixtures of multifunctional acrylate monomers and inorganic filler are cured in the mouth using a light-pipe from a blue light source. This system is more controllable than catalyzed thermal curing and more acceptable to patients and dentists than a UV source. This market is rapidly adopting the new LED sources since they became available a few years ago. In the last year or two, the brightness of these LEDs has been increasing rapidly and the wavelength is moving down. Blue and violet LED lasers are also just becoming available.
UV-induced free radical polymerization is generally catalyzed by aromatic dyes, such as Michler's ketone which transfers its excited state energy to a peroxide co-catalyst that generates free radicals. The dental blue-light curing system, developed by 3M, is based on camphorquinone as a free radical generating curing agent for mixture of methacrylate monomers. Triethanolamine is sometimes used as a co-catalyst to accelerate the reaction. In common with other free radical curing, this reaction is inhibited by oxygen.
A third widely-used UV curing system is based on photo-acid generators in which epoxy-anhydride systems cure by release of acid. The system is particularly attractive because it is not affected by oxygen. At this point there is no widely-used blue version of this chemistry.
UV curable resins are widely used for coating applications, they are used as photoresists for printed circuit boards and integrated circuits and for adhesives where a heat cure would damage the product or where heat is difficult to apply. One adhesive example is the sealing of windshields and rear windows into automobiles. UV curing is also used in some high performance printing inks.
UV cured adhesives are not widely used in the textile industry, although various other treatments including printed inks may be UV cured. Hot melt adhesives are widely used to attach linings and for bonding of fibers in some non-wovens. These adhesives have not been used as a substitute for stitching probably because it is difficult to deliver these viscous materials in the form of small strong dots connecting two fabrics.
What is needed, therefore, are techniques for using adhesives to provide a line of small junction points between two fabrics, equivalent to stitching. The combination of the new small intense blue light sources with a system for delivering small isolated dots of curable monomers makes possible an adhesive system equivalent to a sewing machine. Curing times are optimally of a few seconds or less in order that the process can occur at reasonable speeds. The light source should be in the visible region in order to allow the operator to see it and use the equipment without danger of eye damage.